Absorption of IR Light
• Absorption of IR light causes changes in the vibrational motions of a
molecule.
• The different vibrational modes available to a molecule include stretching
and bending modes.
• The vibrational modes of a molecule are quantized, so they occur only at
specific frequencies which correspond to the frequency of IR light.
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Bond Stretching and Bending
• When the frequency of IR light matches the frequency of a particular
vibrational mode, the IR light is absorbed, causing the amplitude of the
particular bond stretch or bond bend to increase.
2
Characteristics of an IR Spectrum
• In an IR spectrometer, light passes through a sample.
• Frequencies that match the vibrational frequencies are absorbed, and the
remaining light is transmitted to a detector.
• An IR spectrum is a plot of the amount of transmitted light versus its
wavenumber.
• Most bonds in organic molecules absorb in the region of 4000 cm−1 to 400
cm−1.
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Characteristics of an IR Spectrum
• The x-axis is reported in frequencies using a unit called wavenumbers ().
• Wavenumbers are inversely proportional to wavelength and reported in
reciprocal centimeters (cm–1).
• The y-axis is % transmittance: 100% transmittance means
that all the light shone on a sample is transmitted and none
is absorbed.
• 0% transmittance means that none of the light shone on the sample is
transmitted and all is absorbed.
• Each peak corresponds to a particular kind of bond, and
each bond type (such as O − H and C − H) occurs at a characteristic
frequency.
• Infrared (IR) spectroscopy is used to identify what bonds and what
functional groups are in a compound.
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Regions of an IR Spectrum
• The IR spectrum is divided into two regions: the functional group region
(at  1500 cm−1), and the fingerprint region
(at < 1500 cm−1).
Figure 13.9
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Bonds and IR Absorption
• Where a particular bond absorbs in the IR depends on bond
strength and atom mass.
• Stronger bonds (i.e., triple > double > single) vibrate at a higher
frequency, so they absorb at higher wavenumbers.
• Bonds with lighter atoms vibrate at higher frequency, so they
absorb at higher wavenumbers.
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Bonds and IR Absorption
• Bonds can be thought of as springs with weights on each
end (behavior governed by Hooke’s Law).
• The strength of the spring is analogous to the bond
strength, and the mass of the weights is analogous to
atomic mass.
• For two springs with the same weight on each end, the
stronger spring vibrates at a higher frequency.
• For two springs of the same strength, springs with
lighter weights vibrate at a higher frequency than
those with heavier weights.
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Hooke’s Law
• Hooke’s Law describes the relationship of frequency to mass and bond
length.
Figure 13.10
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Four Regions of an IR Spectrum
• Bonds absorb in four predictable regions of an IR spectrum.
Figure 13.11
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10
Bond Strength and % s-Character
• Even subtle differences that affect bond strength affect the frequency of
an IR absorption.
• The higher the percent s-character, the stronger the bond and the higher
the wavenumber of absorption.
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Symmetry and IR Absorption
• For a bond to absorb in the IR, there must be a change in dipole moment
during the vibration.
• Symmetrical nonpolar bonds do not absorb in the IR. This type of
vibration is said to be IR inactive.
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IR Absorptions in Hydrocarbons
• Hexane has only C−C single bonds and sp3 hybridized
C atoms.
• Therefore, it has only one major absorption at 3000-2850 cm−1.
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IR Spectrum of 1-Hexene
• 1-Hexene has a C=C and Csp2−H, in addition to sp3 hybridized C atoms.
• Therefore, there are three major absorptions: Csp2−H
at 3150−3000 cm−1; Csp3−H at 3000−2850 cm−1; C=C at
1650 cm−1.
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IR Spectrum of 2-Butanol
• The OH group of the alcohol shows a strong absorption at 3600-3200
cm−1.
• The peak at ~ 3000 cm−1 is due to sp3 hybridized C−H bonds.
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IR and Structure Determination
• IR spectroscopy is often used to determine the outcome of a chemical
reaction.
• For example, oxidation of the hydroxy group in compound C to form the
carbonyl group in periplanone B is accompanied by the disappearance of
the OH absorption, and the appearance of a carbonyl absorption in the
IR spectrum of the product.
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Using MS and IR for Structure Determination
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